1. Field of the Invention
The present invention relates to a semiconductor film which is formed of gallium nitride (GaN) or a nitride mixed with other metals other than gallium (Ga), and a method of forming the same. The present invention relates to an electronic or an optoelectronic device including the semiconductor film and a method of forming the same. A technical field of the present invention may be broadly defined as a semiconductor thin film structure for forming a high-quality nitride semiconductor thin film on a substrate, and a method of forming the same.
2. Discussion of Related Art
Nitride semiconductors having group III to V elements in the periodic table already occupy an important place in the field of electronic or optoelectronic devices, which will become more important in the future. An application field of nitride semiconductors actually covers a wide range from laser diodes to transistors capable of operating at a high temperature and a high frequency. Also, the application field includes an ultraviolet photodetector, an elastic surface-wave device and a light-emitting diode (LED).
For example, gallium nitride (GaN) is widely known as a material suitable for application to blue LEDs or high-temperature transistors, but is not limited thereto. GaN is being widely studied for microelectronic devices. Also, as will be described hereinafter, GaN may be widely used when included in GaN alloys, such as aluminum gallium nitride (AlGaN), indium gallium nitride (InGaN) and aluminum indium gallium nitride (AlInGaN).
In devices using the nitride semiconductor such as the GaN, a substrate frequently used for growing a nitride semiconductor thin film is a hetero-substrate such as sapphire, silicon carbide (SiC), or silicon. However, because the hetero-substrate has a lattice constant mismatched with and a coefficient of thermal expansion different from a nitride, the nitride semiconductor thin film grown on the hetero-substrate has a number of dislocations and thereby shows problems such as cracking and warpage.
A main technique used in fabricating a GaN photoelectric element and the microelectronic device is to grow a GaN thin film having a low defect density. For this, ‘a two-step growth method’, which includes forming a GaN buffer film at a low temperature and forming a GaN epitaxial film at a high temperature, is mainly used to overcome a mismatched lattice constant between a sapphire substrate and the GaN and to grow a high-quality GaN crystal. In this case, the low temperature GaN buffer film may lower the number of threading dislocations generated due to the mismatched lattice constant, to an extent of 109/cm2. However, stress and warpage due to the difference of the coefficient of thermal expansion between the GaN epitaxial film and the sapphire substrate still remain as problems.
Lately, GaN has been actively studied in applications to a white LED for illustration. In order for the white LED for illumination to really enter the LED market, a price of a white LED chip needs to be significantly lowered. Competition for scaling up the white LED has begun all over the world. The scaling up of the white LED may also be realized by enlarging a production scale of an LED using a sapphire substrate of a large diameter. However, as a diameter of the sapphire substrate becomes larger and larger, a thickness of the sapphire substrate needs to be thickened to prevent the sapphire substrate from being bent in subsequent processes. A warpage phenomenon of the sapphire substrate, as described above, may be caused by the difference of the coefficient of thermal expansion between the GaN and the hetero-substrate. As the thickness of the sapphire substrate becomes larger and larger, the sapphire substrate become less and less bent. As presently expected, the sapphire substrate is required to have a thickness of about 1 mm to about 1.3 mm with respect to a diameter of 6 inches.
Because the sapphire substrate has a coefficient of thermal expansion larger than that of GaN, the GaN epitaxial film may have compressive stress therein when the GaN is grown at a high temperature and then is cooled at a low temperature. Because the silicon substrate has a coefficient of thermal expansion less than that of GaN, the GaN epitaxial film may have tensile stress therein when the GaN is grown at a high temperature and then is cooled at a low temperature. If the stresses are properly lowered, warpage of a substrate may also be reduced. That is, if stresses applied to a GaN film are relieved, a thickness of the substrate may be reduced with respect to the same diameter of the substrate. For example, the sapphire substrate having a thickness of about 500 μm may be used instead of about 1 mm with respect to the diameter of 6 inches. After fabricating the LED, in order to separate the LED chips, considering that the substrate is left to the extent of a thickness of about 100 μm and the remaining thickness of the substrate is removed, as the substrate becomes thinner and thinner, the thinned substrate may provide a large benefit to a productive aspect of the LED.